Synthesis, microstructure and properties of MoAlB ceramics prepared by in situ reactive spark plasma sintering

The renewed research interest on MoAlB ceramic is motivated by its high temperature strength and high temperature oxidation resistance. Bulks were obtained by a reactive spark plasma sintering starting from elemental powders of Mo, Al, and B. Chemical reactions were characterized at intermediate temperatures to define reaction mechanism. The results showed that MoAlB resulted from the reaction of Al8Mo3, MoB, and B or Al8Mo3, Mo, and B at 1000 degrees C. A dense bulk MoAlB ceramic, with a density of 6.26 g/cm(3) (relative density of 97.1%, MoB impurity of 2.0 vol.%), was consolidated at 1200 degrees C for 10 min in vacuum. Microstructure, physical and mechanical properties were also reported. The average grain size was 14.5 mu m in length and 6.9 mu m in width. The measured thermal expansion coefficient and thermal conductivity were 10.1 x 10(-6) K-1 and 37.56 W.m(-1).K-1, respectively, and the electrical conductivity was determined as 2.70 x 10(6) S/m at room temperature. The flexural strength, fracture toughness, and compressive strength were 465.8 MPa, 5.45 MPa.m(1/)(2), and 1467.6 MPa, respectively. Vickers hardness decreased from 13.95 to 9.79 GPa with the increasing load in the range of 10-300 N at ambient temperature. Most significantly, the high temperature hardness could be retained up to 700 degrees C without any degradation, exhibiting excellent high temperature stiffness.